As semiconductor devices scale to smaller dimensions, a need has arisen to develop three dimensional structures to provide more device functionality within a given area on a substrate. Examples of these three dimensional structures include finFET logic devices as well as 3 dimensional (3-D) memory devices. One example of 3D memory attracting recent interest is a type of flash memory known as a vertical NAND device or VNAND. In some particular implementations, the VNAND devices are implemented by fabricating 16, 32, or 64 layers to serve as storage elements. In one variant, in order to access different layers the VNAND device stack may be patterned with a masking element, and subsequently etched in a series of cycles through the series of layers contained in the device stack. In a given cycle, the mask size is reduced by resist trimming between successive etch operations to form a pyramidal staircase structure where the top of the pyramid may be the top device layer, and the base of the pyramid constitutes the deepest device layer exposed so contact formation can subsequently be made. This process may entail many cycles including many etch operations, because the etch operation is performed as a vertical reactive ion etching sequence configured to etch just one or a few layers in a cycle. Thus, for a 64 layer device stack, a series of 8, 16, or greater number of etch operations may be performed, as well as a like number of mask trimming operations. In addition, after performing such a large number of etch operations, the shape of such device structures may be compromised, leading to a less reliable process. Moreover, the resist trimming operation may result in reduction in dimension of a mask element along all sides of a square or rectangular resist shape, resulting in a square pyramid device structure occupying a large projected area on the substrate. An overly large structure is produce even in the case when just two sides of the pyramid structure are to be accessed to form a memory device.
It is with respect to these and other considerations the present improvements have been needed.